1. Field of the Invention
The present invention relates generally to the field of logging of material properties and, in one possible specific embodiment, relates to methods and/or devices for making a log in layered environments. One possible non-limiting example includes producing a log of material properties with respect to borehole depth.
2. Description of the Background
Subsurface geological formations typically comprise layers of various types of formations. While the present invention is not limited to use in producing logs of a layered environment comprising subsurface geological formations, an embodiment of the invention is conveniently described in terms of this environment.
Most oil and gas was originally deposited in an ocean environment. As a consequence, such formations may contain fluids such as salt water and/or oil. Salt water, with its mobile sodium and chlorine ions makes the formation conductive to electricity, while the oil/gas makes the formation resistive. The oil companies typically utilize logging tools to produce a log of material properties of a wellbore. As one example, when the desired rock formation or depth is reached, the drill pipe and the bit are removed from the hole. An instrument is lowered into the wellbore to measure the electrical conductivity versus depth. In this way, a log or a record of the geologic formation is produced. Other instruments may generate a log of a wellbore while drilling. Generally, if the rocks are relatively conductive, they contain salt water. If the rocks are relatively resistive, they contain oil and/or gas.
The earliest instruments used direct current and were first used in 1927. In the 1950's, electromagnetic or induction tools were introduced. These electromagnetic instruments had coaxial coils, and measured just one component of the conductivity tensor of the rock. There are many different electromagnetic tools which measure various physical quantities. The standard induction tools measure a voltage while the measurement-while-drilling (MWD) tools measure phase differences and/or amplitude ratios. Other tools comprise many configurations such as laterolog tools, normal and lateral tools, e-log tools and the like. The present invention may be utilized with these and other tools.
Oil is often deposited in a layered environment. There is an exact mathematical solution to an electromagnetic instrument penetrating a parallel layered environment at any angle as per an article by the inventor. See, for example, Hardman and Shen, “Theory of Induction Sonde in Dipping Beds,” Geophysics Vol. 51, No. 3, March 1986, p. 800-809. However, in the real world, the interface between the layers is not necessarily parallel.
Other background material may include Hardman and Shen, “Charts for Correcting Effects of Formation Dip and Hole Deviation on Induction Logs,” The Log Analyst, Vol. 28, No. 4, p 349-356, July-August 1987; Hardman, “Four-Term Decomposition Techniques for a Faster Inversion of Induction Responses,” SPE 84606, October 2003; Wang, Barber, et al., “Triaxial Induction Logging; Theory, Modeling, Inversion, and Induction,” SPE 103897, December 2006; Anderson, Barbara et al., “Effect of Dipping Beds on the Response of Induction Tools”, SPE Formation Evaluation (March 1988), pp. 29-36; Barber, Anderson, et al, “Determining Formation Resistivity Anisotropy in the Presence of Invasion, SPE 90526, September 2004; Anderson, Barbara et al., “Response of 2-MHZ LWD Resistivity and Wireline Induction Tools in Dipping Beds and Laminated Formations”, SPWLA 31st Annual Logging Symposium, Jun. 24-27, 1990, Paper A, pp. 1-25; Barber, Thomas D. et al., “Interpretation of Multiarray Induction Logs in Invaded Formations at High Relative Dip Angles”, The Log Analyst, vol. 40, No. 3 (May-June 1999), pp. 202-21; Sommerfeld Partial Differential Equations in Physics, Academic Press 1949; U.S. Pat. No. 3,808,520; U.S. Pat. No. 6,304,086; U.S. Pat. No. 6,573,722; U.S. Pat. No. 6,216,089; U.S. Pat. No. 3,510,757; US 2006/0038571; US 2007/0256832; US 2003/0222651; US 2003/0146753; US 2003/0155924; US 2005/0127917; US 2004/0017197; US 2006/0192562; US 2003/0146751; US 2009/0018775; US 2008/0078580; US 2008/0210420; US 2008/0215241; US 2008/0258733; US 2008/0078580; US 2008/0278169; and US 2005/0256642.
Since around 2000, the tools have transmitter and receiver coils in the x, y and z directions. These tri-axial instruments measure all the components of the conductivity tensor and are able to orient the individual bed boundaries. A change in bed boundary orientation may be indicative of a change in the depositional environment. Information concerning the orientation of the bed boundary may be very useful in the geologic interpretation of the formation.
Consequently, there remains a long felt need for improved methods which may be utilized to produce more accurate logs in layered environments wherein the layers may or may not be parallel. Moreover, it sometimes desirable to more quickly calculate or invert logs. Because those skilled in the art have recognized and attempted to solve these problems in the past, they will appreciate the present invention, which addresses these and other problems.